Inkjet printing mechanisms use pens which shoot drops of liquid colorant, referred to generally herein as "ink," onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, each printhead is propelled back and forth across the page by a carriage assembly, with each printhead shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a "service station" mechanism is mounted within the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. To facilitate priming, some printers have priming caps that are connected to a pumping unit to draw a vacuum on the printhead. During operation, partial occlusions or clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a clearing or purging process known as "spitting." The waste ink is collected at a spitting reservoir portion of the service station, known as a "spittoon." After spitting, uncapping, or occasionally during printing, most service stations have a flexible wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead.
To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide quicker, more waterfast printing with darker blacks and more vivid colors, pigment based inks have been developed. These pigment based inks have a higher solids content than the earlier dye-based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to use plain paper.
Other research has focused on increasing printing resolutions, by decreasing the size of the ink droplet. Smaller ink droplets must be more accurately positioned on the print media to reproduce the desired image. Thus, ink drop placement requirements for high-quality images has become a very exacting art. Unfortunately, small, regularly repeated deviations from the expected drop placement can cause image striping that is unacceptable and degrades the print quality. These repeated droplet deviations may be caused by variations in the velocity of the printhead as it scans across the page, as well as from drop firing direction errors. The invention claimed below addresses the first of these problems, periodic variations in the velocity of the printhead. One common source of these periodic variations in the printhead velocity was found to be vibrations induced in the carriage by the energy efficient, economical electric motor used to drive the printhead carriage back and forth across the page.
One earlier solution to the problem of isolating the moving printhead from the vibrations induced by the carriage motor consisted of inserting a metal compression spring between the drive belt which couples the motor with the carriage. Unfortunately, use of this metal compression spring had several limitations. First, if relative motion between touching parts is required, such as between features used to retain the system to the carriage, any friction caused by the metal compression spring can keep these parts from freely moving, so no vibration isolation is achieved. This is often the case for the very small vibrations caused by the carriage drive motor. Second, space limitations within a compact printer may make it difficult to locate the metal springs into the available space. A third disadvantage of the compression spring solution is that adding any damping to the metal spring requires the addition of another part to the printer, increasing the part cost as follows assembly costs. Finally, if the printhead vibration is not in the direction of the compression spring, vibration isolation is never achieved.
Another solution for isolating these periodic vibrations caused by the carriage motor consisted of placing a damping material, such as a foam or rubber pad, between the motor and the motor mount on the chassis. While this solution isolated the vibrations transferred from electric motor to the printer chassis, it failed to directly isolate the carriage assembly from motor vibrations.
Thus, a need exists for isolating the printhead carriage from periodic vibrations induced by the carriage drive motor, to eliminate undesirable stripe marks for appearing on the printed image.